Role of Proteins in Insulin Secretion and Glycemic Control - Advances in Food and Nutrition Research - page 9

The amino acid composition of protein foods differ markedly
( Table 1.1 ). Animal proteins have a more balanced amino acid profile and
contain notable amounts of both essential and nonessential amino acids.
Plant proteins, on the other hand, often show deficiencies in certain essential
amino acids. For example, cereals and legumes are generally deficient in
lysine and methionine, respectively ( Jeor et al., 2001 ). Some nonessential
amino acids such as glutamic acid and aspartic acid are abundantly found
in all proteins.
4. IMPACT OF PROTEINS AND AMINO ACIDS
ON INSULIN SECRETION
4.1. Effect of proteins on insulin secretion
In early work, Floyd, Fajans, Conn, Knopf, and Rull (1966b) and Berger
and Vongaraya (1966) showed that proteins and amino acids significantly
increased insulin expression. Since then, many studies have confirmed that
both ingested proteins and intravenous administration of amino acids stim-
ulate insulin secretion ( Fajans, Floyd, Knopf, & Conn, 1967; Gannon &
Nuttall, 2010 ) .
Proteins are believed to induce insulin secretion both by the direct stim-
ulation of pancreatic b -cells by amino acids and via incretin hormones
expressed in response to meal composition ( Nuttall & Gannon, 1991 ) . Early
in vitro work using pieces of rabbit pancreas incubated in media containing
amino acids showed that insulin is stimulated in the presence of amino acids
( Milner, 1970 ). This was better demonstrated by Salehi et al. (2012) using
mouse pancreatic islets. The authors fed either whey protein or white bread
to human subjects and observed insulin expression by mouse islets when
they were exposed to the resulting serum. The results showed that amino
acids in serums increased insulin expression. The mechanisms by which
amino acids stimulate insulin appear to be different to that of glucose.
In an in vitro experiment utilizing b HC9 hyperplastic insulin secreting cells,
Ronner, Naumann, and Friel (2001) demonstrated that amino acids stimu-
late insulin also using different mechanisms to glucose. They proposed a
two-compartment model where insulin secretion occurs in response to an
amino acid sensor in the b -cells and when K ATP channels have low activity.
This suggested that amino acid-mediated insulin secretion does not occur
when b -cells are poorly energized and when glucose concentrations are low.
Circulating plasma amino acid levels also have effects on peripheral glu-
cose uptake mechanisms. Increased plasma amino acid levels have been